GPS receiver with improved immunity to burst transmissions

ABSTRACT

GPS signals are typically weak and thus easily interfered with by other radio transmissions in the same or adjacent frequency bands. Interference can be especially problematic when the GPS receiver is co-located with a communications device that includes a radio transmitter, such as a cellular telephone. The transmitted signal from the co-located communication device can overload (or saturate) the GPS receiver front-end designed to receive weak GPS signals. In such a situation no useful information can be extracted from the received GPS signals originating from the GPS satellites. Described herein is a novel apparatus and method that can be used to minimize the effect of co-located interference on a GPS receiver.

PRIORITY CLAIM

[0001] This application claims the benefit of U.S. application Ser. No.10/147,983 filed May 20, 2002, which is hereby incorporated by referencein its entirety.

FIELD OF THE INVENTION

[0002] The invention relates to spread-spectrum communications and, inparticular, to an improved GPS receiver in close proximity to a radiofrequency transmitter.

BACKGROUND

[0003] The basic functionality of a Global Positioning System (GPS)receiver is to calculate the latitude, longitude and altitude of the GPSreceiver's location (i.e., the co-ordinates of the receiver) uponreceiving a number of GPS signals from a network of GPS satellites thatorbit the earth. The calculation of the co-ordinates of the GPS receivertypically begins by comparing the timing associated with a select numberof received GPS signals. After the initial comparison of the receivedGPS signals, values for timing corrections associated with the selectgroup of received GPS signals are established. The timing correctionsare made in order to solve a three-dimensional geometric problem, whichhas as its solution the co-ordinates of the GPS receiver.

[0004] The received GPS signals are typically weak and thus easilyinterfered with by other radio transmissions in the same or adjacentfrequency bands. Interference can be especially problematic when the GPSreceiver is co-located with a communications device that includes aradio transmitter, such as a cellular telephone. The transmitted signalfrom the co-located communication device can overload (or saturate) theGPS receiver front-end designed to receive weak GPS signals. In such asituation no useful information can be extracted from the received GPSsignals originating from the GPS satellites.

[0005] In select instances this problem may be overcome by filtering allof the received signals from the GPS antenna before down conversion ofthe respective transmission signal band by the GPS receiver front-end.Typically a low noise amplifier (LNA) is first used to amplify thesignal before further filtering or mixing to another frequency. Theresult of adopting this approach is that the loss of all signal energiesin the filter reduces the sensitivity of the GPS receiver permanently,irrespective of whether or not the co-located communications device istransmitting. This is undesirable as the GPS signals received from thesatellites are weak and reducing the sensitivity of the GPS receiverfurther reduces the operability of the system. Additionally, the filterwould also occupy space, add cost to the unit and would draw additionalpower.

[0006] The problem caused by the co-located communications device may bealternatively overcome by the use of a high linearity LNA. This wouldensure that the LNA is capable of amplifying the GPS signal despite thepresence of a large interfering locally generated transmission. Thedisadvantage of this solution is that such an LNA would consumeadditional power, which is not acceptable in a portable battery powereddevice such as a cellular telephone. A filter following the LNA wouldalso be required to provide sufficient rejection of the interferingsignal to prevent overload of the next stage of the receiver, typicallya mixer. These additional performance requirements increase the size,power consumption and cost of the filter and make implementing a highlyintegrated receiver design without the additional filter difficult.

[0007] Given that it is not easily possible to remove the effect of theinterfering transmission, it is important to achieve the maximumperformance possible despite the interference. A method of achievingthis that has been commonly used is to employ a ‘blanking’ signal,derived from the transmitter of the co-located communications device andactive whenever that transmitter is switched on, which is used tosuppress the operation of the GPS receiver during the transmission. Thedisadvantage of this is that such a signal is not always easily derivedfrom the co-located transmitter. Even if such a signal can be derivedfrom the co-located transmitter, the physical construction of the unitmay preclude the connection of the signal into the GPS receiver. Forexample, the GPS receiver and the communications device, whileco-located, may not be physically constructed as a single unit.Furthermore, there may be more than one communications device, such as acellular telephone with additional functions such as a short-range radiolink.

[0008] Under these circumstances, it would be advantageous if the GPSreceiver can determine for itself the period during which a co-locatedtransmitter is active and take such action as to mitigate as far aspossible the loss of performance caused by the interfering transmission.

SUMMARY OF THE INVENTION

[0009] The invention may be summarized according to one aspect as amethod of limiting the effect of interfering transmission on a GPS(Global Positioning System) receiver, the GPS receiver having a radiofront-end and a radio back-end, the radio front-end performingdown-conversion of at least one GPS radio signal received at a RadioFrequency (RF) to an Intermediate Frequency (IF), and the radio back-endderiving a bit stream of digital data from the at least one GPS radiosignal after it has been down converted to the IF and processing thebit-stream of digital data, the method comprising the steps of: i)sensing an overload condition in the radio front-end when the receivedradio signal is above a threshold; ii) generating an overload signalupon sensing the overload condition of the radio front-end; iii)coupling the overload signal into the radio backend; and iv)substituting in the radio back-end the bit-stream of digital data with alocally generated bit pattern in response to the presence of theoverload signal, the locally generated bit pattern being selected suchthat when processed it causes less noise to accumulate in the radioback-end than if the bit-stream of digital data were processed.

[0010] According to another aspect the invention provides a GPS (GlobalPositioning System) receiver comprising a radio front-end and a radioback-end, the radio front-end performing down-conversion of at least oneGPS radio signal received at a Radio Frequency (RF) to an IntermediateFrequency (IF), and the radio back-end deriving a bit-stream of digitaldata from the at least one GPS radio signal after it has been downconverted to the IF and processing the bit-stream of digital data, anoverload detector for generating an overload signal in the radiofront-end when the received radio signal is above a threshold andsending the overload signal to the radio back-end; and means forsubstituting the bit-stream of digital data with a locally generated bitpattern in response to the presence of the overload signal, the locallygenerated bit pattern being selected such that when processed it causesless noise to accumulate in the radio back-end than if the bit-stream ofdigital data were processed.

[0011] According to another aspect the invention provides a GPS (GlobalPositioning System) receiver comprising a radio front-end and a radioback-end, the radio back-end deriving a bit-stream of digital data fromat least one receiver GPS radio signal and processing the bit-stream ofdigital data, an overload detector for generating an overload signal inthe radio front-end when the received radio signal is above a thresholdand sending the overload signal to the radio back-end; and a means forsubstituting the bit-stream of digital data with a locally generated bitpattern in response to the presence of the overload signal, the locallygenerated bit pattern being selected such that when processed it causesless noise to accumulate in the radio back-end than if the bit-stream ofdigital data were processed.

[0012] According to another aspect the invention provides a method oflimiting the effect of interfering transmission on a GPS (GlobalPositioning System) receiver, the GPS receiver having a radio front-endand a radio back-end, the radio back-end deriving a bit-stream ofdigital data from at least one GPS radio signal and processing thebit-stream of digital data, the method comprising the steps of: i)sensing an overload condition in the radio front-end when the receivedradio signal is above a threshold; ii) generating an overload signalupon sensing the overload condition of the radio front-end; iii)coupling the overload signal into the radio back-end; and iv)substituting in the radio back-end the bit-stream of digital data with alocally generated bit pattern in response to the presence of theoverload signal, the locally generated bit pattern being selected suchthat when processed it causes less noise to accumulate in the radioback-end than if the bit-stream of digital data were processed.

[0013] Other aspects and features of the present invention will becomeapparent, to those ordinarily skilled in the art, upon review of thefollowing description of the specific embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Preferred embodiments of the invention will now be described ingreater detail with reference to the accompanying diagrams, in which:

[0015]FIG. 1 is a block diagram illustrating a conventionalSuperheterodyne (superhet) GPS receiver;

[0016]FIG. 2 is a block diagram a superhet GPS receiver improvedaccording to aspects of the invention;

[0017]FIG. 3 is a schematic representation of a typical circuit forsensing a signal level; and

[0018]FIG. 4 is a schematic representation of an embodiment of a datamodifier circuit according to aspects of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The Superheterodyne (superhet) Architecture is a common receiverfront-end architecture used for mobile communication applications. Shownin FIG. 1 is a simplified block diagram of a conventional GPS receiver100 incorporating the superhet architecture. In FIG. 1 the superhetarchitecture comprises an antenna 102, a Low Noise Amplifier (LNA) 104,an optional image-rejection filter 106, a mixer 108, a VoltageControlled Oscillator (VCO) 100 and an Intermediate Frequency (IF)filter 112.

[0020] A radio signal 20 at the radio frequency (RF) is first receivedby the antenna 102 and amplified by the LNA 20. The filter 106 thenoptionally filters the radio signal 20. The requirement for this filtercan be removed by integrating its function into the LNA 20 or the mixer108. The mixer 108 down-converts the radio signal 20 from the RF to thelower IF by using a local oscillator signal LO generating by the VCO110. At this point the radio signal 20 is now centered on a low enoughfrequency where it is possible to perform the back-end processing.

[0021] The transition from front-end to back-end processing requires theradio signal 20 that has been down-converted to be filtered by the IFfilter 112 and then passed to an analogue-to-digital converter (ADC)114. The ADC 114 converts the radio signal 20 from a summation ofanalogue waveforms into a bit-stream of digital data that can beprocessed.

[0022] The back-end processing of received signals that is of concern tothe present invention comprises a correlator 116 and a processor 118.GPS signals are modulated in a manner similar to CDMA transmissions,whereby pseudo-random codes are employed to identify each of theorbiting GPS satellites and aid in resolving the timing of the receivedGPS signals. As such the signals are processed digitally by logic whichperforms a correlation function in the correlator 116. In the correlator116 the processing gain applied to the spread spectrum GPS signal raisesthe signal level above the noise being received when a local copy of thecode specific to individual GPS satellite is placed in-phase with thereceived signal modulated with the same code. This correlation andsynchronization process must be carried out for all the GPS satellitesin order to identify the strongest set of signals to be used tocalculate the GPS receiver's co-ordinates. The results are then passedfrom the correlator 116 to the processor 118 to determine the signalpath delays to each of the satellites and hence enable calculation ofthe location of the receiver.

[0023] When a large interference signal is present, such as from aco-located or nearby transmitter, the front-end radio circuits becomeoverloaded and the GPS signal is corrupted, preventing the correlator116 block from obtaining useful information. As the correlator 116cannot determine that the GPS signal is corrupted it continues toprocess the received signal 20, continuing to accumulate noise, so thatthe ratio of useful signal to noise (SNR) is reduced.

[0024] When operating under normal signal conditions the correlated GPSsignal accumulates linearly with duration, while the noise accumulateswith the square root of the duration, so increasing the durationimproves the desired signal-to-noise ratio (SNR). When the system isoverloaded the GPS signal is corrupted and the desired signalaccumulation ceases while the noise accumulation continues to increase,resulting in a degradation of the signal-to-noise ratio.

[0025]FIG. 2 is a simplified block diagram showing how the conventionalGPS receiver 100 can be modified to provide an improved GPS receiver 200according to aspects of the invention. The GPS receiver 200 also usesthe superhet front-end architecture that was employed in the GPSreceiver 100 of FIG. 1. Therefore an antenna 202, LNA 204, an optionalfilter 206, a mixer 208, a VCO 210 and an IF filter 212 all have thesame basic functionality as the corresponding circuits 102, 104, 106,108, 110 and 112 respectfully of the GPS receiver 100, shown in FIG. 1.The back-end of the GPS receiver 200 of FIG. 2 also includes an ADC 214,correlator 216 and a processor 218 corresponding respectively to the ADC214, the correlator 116 and optionally a processor 218 shown in FIG. 1.

[0026] In addition to the aforementioned components, the GPS receiver200 is improved by enabling the radio front-end to detect stronginterfering signals and pass that information to the radio back-end. Tothis end a further circuit referred to as a overload detector 211 isconnected to or is integrated into the mixer 208 and provides anelectronic signal 40 hereinafter referred to as the overload signal. Theoverload signal 40 is then routed to the radio back-end. In the presentembodiment of the invention, as shown in FIG. 2, a further circuit inthe radio back-end referred to as a data modifier 215 accepts at acontrol input 291 the overload signal 40. The data modifier 215 isconnected between the ADC 214 and the correlator 216, and receives atits data input 290 the output of the ADC 214.

[0027] More precisely, the overload signal 40 is derived from a point inthe chain of circuits processing the radio signal (i.e. the radiofront-end) which indicates that a large signal is present and theoverload is at such a level that the GPS signal will be corrupted at theoutput of the ADC 214, preventing the system from obtaining usefulinformation at this time from the correlator 216. This signal is shownas being derived from the mixer 208 in a overload detector 211 circuitbut it could be derived from any of the front-end radio blocks where itis possible to distinguish the level of signal being received. Forexample, the overload signal 40 may be derived from the LNA 204 as afunction of the linearity of operation of the LNA 204. Preferably, theoverload detector 211 is integrated into the mixer 208 or any of theother front-end radio circuits. However, the overload detector 211 hasbeen shown here as a separate block in order to discuss its function andhighlight its presence in the radio front-end.

[0028] The data modifier 215 is meant to selectively substitute theoutput of the ADC 214 with a digital bit pattern constructed so as toprevent the correlator 216 from accumulating noise while the GPS signalis corrupted. The simplest sequence of bit values that accomplishes thisis an alternating series of +1 and −1 values, though other patterns arepossible. The values of +1 and −1 are often represented on the digitaloutput as logic 1 and logic 0 respectively, though again otherrepresentations are possible which achieve the same purpose.

[0029] The replacement data stream has the desired property that whenaccumulated for a period of many data bits the average valuesubstantially tends rapidly towards zero with a zero value standarddeviation, while the random data stream normally present in the overloadconditions tends towards zero but with a standard deviation proportionalto the square root of the number of bits accumulated. This results inthe noise accumulation in the correlator ceasing for the duration of theoverload, so the desired signal-to-noise ratio does not decrease due tothe overload condition.

[0030] Note that the digital data stream is often processed before beingused by the correlator, for instance multiplying by the output of anumerically controlled digital oscillator to remove the intermediatefrequency by down conversion to baseband. This type of processing doesnot change the statistical properties of the data stream regarding theaccumulation of noise.

[0031] The overload signal 40 could be passed into a control input ofthe correlator block directly to provide another embodiment of theinvention in which the correlator 216 directly accepts at a data inputthe output of the ADC 214. In such a case, there would be no need for anexplicit data modifier 215 circuit, as the functionality of the datamodifier 215 would be integrated into the correlator 216. However suchan embodiment might not be practical as it is often the case that theradio circuits and the correlator circuits are built as separateintegrated circuits making it difficult or impossible to couple anoverload signal to the correlator.

[0032]FIG. 3 shows a typical embodiment of the overload detector 211.Assuming a differential signal path from the mixer 208, the level of theradio signal 30 is peak-detected by the action of a differential pair ofbipolar junction transistors Q1 and Q2, each BJT having a base terminalb1 and b2 respectively. The base terminals b1 and b2 serve as thedifferential inputs to the overload detector 211 receiving adifferential signal Vin from the mixer 208. The overload detectorfurther comprises a resistor R3 and capacitor C connected in parallelbetween a common emitter node 60 and a ground (GND) voltage rail.Furthermore, the collector of each BJT Q1 and Q2 is connected to a powersupply rail Vcc via transistors R1 and R2, respectively. Lastly, theemitter node 60 is connected to a comparator 70, the comparator alsohaving a second input connected to a voltage reference VREF and anoutput from which the overload signal 40 can be tapped.

[0033] The overload detector 211 is actually a basic embodiment of an‘envelope detector’ circuit. Such a circuit works by use of large signalswing non-linear operation, so small signal analysis typically employedin analogue circuit design does not explain its operation accurately.The circuit topology can be built up as follows. The basic operation ofthe overload detector 211 is to follow the envelope of anamplitude-modulated signal Vin, removing the carrier and giving anoutput—the overload signal 40—proportional to the amplitude of the radiosignal 30.

[0034] The simplest envelope detector (not shown) is a series diodefeeding a capacitor in parallel with a resistor. The capacitor chargesvia the diode on the signal peaks, and discharges (slowly with respectto the carrier period, rapidly with respect to the modulation period)via the resistor. This simple circuit has a very low input non-linearimpedance (essentially zero on the charging peaks, infinite at othertimes) so the diode is replaced with a transistor. As the input risesthe transistor turns on and charges the capacitor from the supply line,as it then drops the transistor turns off, leaving the capacitor withthe peak voltage (minus a VBE drop, a permanent offset). The inputimpedance is thus higher, though still non-linear.

[0035] The transistor version still only works on one polarity peak, soby having a differential input both positive and negative signal peakscan both be used, which has the benefit of doubling the carrier ripplefrequency, making the choice of RC time constant slightly easier. Theresistance value R has to be chosen considering DC bias conditions aswell as the RC time constant.

[0036] A practical circuit is more complex, as DC variations due toprocess variation, temperature and supply voltage all need compensating,as well as circuits to bias the circuit to work with signal levels smallcompared to a VBE drop.

[0037] During normal operation, i.e., small signal operation of theradio front-end the overload detector 211 receives the differentialinput Vin from the mixer 208. The voltage at the emitter node 60 VEremains at a nominal voltage ensuring that both BJT are operating intheir active mode, i.e., they are turned on. When the nominal voltage iscompared to the reference voltage VREF the derived overload signal 40represents a nominal condition where the radio front-end is not beingoverloaded.

[0038] However, during large signal operation, i.e., overload operationthe differential input Vin received from the mixer 208 causes the BJT'sQ1 and Q2 to switch on and off depending on the phase of the signal atany given instant. The large input signal swing causes the instantaneousbase voltage at any given instance to be very large and in turn forcesthe common emitter node 60 voltage VE to rise such that the base-emittervoltage remains approximately equal to the 0.7 Volt threshold voltageintrinsic to the base-emitter PN junction of the BJT. Once VE risessignificantly in relation to VREF the comparator switches the overloadsignal 40 to indicate that the radio front-end is being overloaded by astrong transmission within the same transmission band or within anadjacent signal band.

[0039]FIG. 4 shows a specific embodiment of the data modifier 215. Thecontrol input 291 of the data modifier is connected to receive theoverload signal from the radio front-end. In this embodiment theoverload signal 40 is the control signal for a 2:1 Multiplexer (MUX) 83.The overload signal 40 can be active high or active low depending uponthe design choices made by one skilled in the art without unnecessaryexperimentation. The MUX 83 has two other inputs A0 and B0, one of whichat any given instant is selectively coupled to the MUX 83 output Z0. Theinput A0 is connected to the data input 290 of the data modifier, whichis externally connected to receive the output of the ADC 214. The MUX 83output Z0 is also the output of the data modifier 215. As shown in FIG.4 the digital output (bit stream) from the ADC 214 is connected to inputA0. The digital output of the ADC 215 is coupled to the output of thedata modifier 215 via the MUX 83 when the radio front-end is notexperiencing overload conditions. However when the radio front-end isexperiencing overload conditions the overload signal 40 will be drivenactive to indicate this fact and the MUX 83 will couple B0 to its outputZ0. Connected to B0 is an overload pattern generator 80. In thisembodiment the overload pattern generator 80 delivers an alternatingseries of logic 1's and 0's. This pattern is generated by coupling theinverting output QN of a D-type latch 85 to its input terminal D andusing the non-inverting output Q as the source of the overload patternto be connected the MUX 83 input B0. Furthermore, the pattern alternatesaccording to a digital clock signal CLK that provides the timing fordigital circuits in the radio back-end.

[0040] The overload detector 211 described above is only one embodimentof a signal detection means that is usable according to aspects of theinvention. Other well known signal detection means could also be used.

[0041] Similarly, although down converting the received radio signalfrom the RF to the IF was a feature used in the embodiment disclosed,the modifications to the GPS receiver according to aspects of theinvention could be employed in a radio receiver that did not have downconversion as a feature. In other words the digital signal processingmay take place at the RF; however, this would not affect the operabilityof the invention disclosed when applied to such a radio receiver.

[0042] The proposed improvement has the additional benefit that it canbe fitted to systems based on existing correlators and processor deviceswithout requiring their modification in order to gain the systemadvantage shown.

[0043] The overload signal 40 can also be used as an input to othercircuit blocks in the receiver, such as automatic gain control circuits,in order to assist the circuit to recover rapidly from the overloadcondition.

[0044] We can determine the benefits to be gained from the presentinvention as follows. Assume for simplicity the transmission sequence asused by the GSM cellular standard, though any time division duplex ortime division multiple access system could be substituted by changingthe various parameters discussed.

[0045] A co-located transmitter is turned on for a burst periodcorresponding to one or more slots in a frame of a preset number ofslots, 8 in the case of GSM. The GSM enhancement known as GPRS allowsthe transmitter to be switched on for 2 or 4 slots rather than the 1slot used normally for voice. The co-located transmitter is therefore onfor a proportion of the time varying from ⅛ to ½ depending on the modeof operation. Defining the proportion of time the transmitter is on tobe f, where f varies from 0 situations previously discussed.

[0046] For GPS receivers without the improvements provided by aspectsaccording to the invention, the GPS system performance is changed by 20log(1−f) dB. Alternatively, for GPS receivers able to benefit from theimprovements provided by aspects of the invention the GPS systemperformance changes by only 10 log(1−f) dB. These values are tabulatedbelow for example values of f. System change of performance (dB) FNormal System Improved System 0 0.0 0.0 0.125 −1.2 −0.6 0.25 −2.5 −1.20.375 −4.1 −2.0 0.5 −6.0 −3.0 0.625 −8.5 −4.3 0.875 −18.1 −9.0

[0047] This demonstrates that even a single slot system will benefit by0.6 dB, and as GPRS systems become more common a benefit of 3 dB willoften occur.

[0048] What has been described is merely illustrative of the applicationof the principles of the invention. Other arrangements and methods canbe implemented by those skilled in the art without departing from thespirit and scope of the present invention.

1. A method of operating a radio receiver, the radio receiver comprisinga radio receiver front-end, the method comprising: sensing an overloadcondition in the radio receiver front-end when a received radio signalis above a threshold; and generating an overload signal in response tosensing the overload condition.
 2. The method of claim 1 furthercomprising: coupling the overload signal into a radio receiver back-end;and coupling a locally generated bit pattern into the radio receiverback-end in response to the presence of the overload signal, the locallygenerated bit pattern being selected such that when processed it causesless noise to accumulate in the radio receiver back-end than if abit-stream derived from the received radio signal were processed.
 3. Amethod according to claim 1, wherein the step of sensing the overloadcondition is carried out by measuring an amplitude envelope of thereceived radio signal.
 4. A method according to claim 1, wherein thestep of sensing the overload condition is carried out by measuring asignal to noise ratio of the received radio signal.
 5. A methodaccording to claim 1, wherein the step of sensing the overload conditionis carried out by measuring a carrier to interference ratio of thereceived radio signal.
 6. A method according to claim 2, wherein thelocally generated bit pattern is an alternating sequence of high and lowbinary values.
 7. A method according to claim 2, wherein the locallygenerated bit pattern is a pseudo random sequence.
 8. A method accordingto claim 2, wherein the locally generated bit pattern has the propertyof an average value tending rapidly towards zero with a substantiallyzero value standard deviation.
 9. A radio receiver front-end forreceiving radio signals, the radio receiver front-end comprising: anoverload detector for generating an overload signal when a receivedradio signal is above a threshold; and an output port connectable to aradio receiver back-end, the output port used for coupling the overloadsignal to the radio receiver back-end.
 10. The radio receiver front-endof claim 9 integrated into a GPS (Global Positioning System) receiver.11. The radio receiver front-end of claim 9 further adapted todown-convert the received radio signal from a Radio Frequency (RF) to anIntermediate Frequency (IF), wherein the received radio signal iscombined with a locally generated RF signal within a mixer to produce adown-converted copy of the received radio signal.
 12. A radio receiverback-end for processing received radio signals, the radio receiverback-end comprising: an input port connectable to a radio receiverfront-end, the input port used for coupling an overload signal into theradio receiver back-end; a converter for deriving a bit-stream ofdigital data from a received radio signal; and a means for substitutingthe bit-stream of digital data with a locally generated bit pattern inresponse to receiving the overload signal, the locally generated bitpattern being selected such that when processed it causes less noise toaccumulate in the radio receiver back-end than if the bit-stream ofdigital data were processed.
 13. The radio receiver back-end of claim 12wherein the converter is an analog-to-digital converter.
 14. The radioreceiver back-end of claim 13 further comprising a correlator andwherein the means for substituting the bit-stream of digital data with alocally generated bit pattern comprises a data modifier having a datainput connected to receive the output of the analog-to-digital converterand the data modifier having a control input connected to receive theoverload signal from the input port, the data modifier generating andsubstituting the locally generated bit pattern for the bit-stream ofdigital data that is input to the correlator when the overload signal isa value that indicates an overload condition.
 15. The radio receiverback-end of claim 12 integrated into a GPS (Global Positioning System)receiver.
 16. A method of limiting the effect of interferingtransmission on a GPS (Global Positioning System) receiver, the GPSreceiver comprising a radio receiver front-end, the method comprising:sensing an overload condition in the radio receiver front-end when areceived radio signal is above a threshold; and generating an overloadsignal in response to sensing the overload condition.
 17. The method ofclaim 16 further comprising: coupling the overload signal into a radioreceiver back-end of the GPS receiver; and coupling a locally generatedbit pattern into the radio receiver back-end in response to the presenceof the overload signal, the locally generated bit pattern being selectedsuch that when processed it causes less noise to accumulate in the radioreceiver back-end than if a bit-stream derived from the received radiosignal were processed.
 18. An overload detector for detecting anoverload condition in a radio receiver, the overload detectorcomprising: an overload detector for detecting an overload condition inthe radio receiver; and an overload signal generator for generating anoverload signal in response to a detected overload condition in theradio receiver.
 19. The overload detector of claim 18 in combinationwith a data modifier, the data modifier coupled to the overload signalgenerator to receive the overload signal, and the data modifier forproviding a locally generated bit stream to the radio receiver when theoverload signal is indicative of the detected overload condition.